How can one determine, or know, if catalytic converters are plugged? I understand that such condition would cause a lot of heat build up in engine bay, but how can one best learn of condition of cats being plugged to some degree?

 

If bad enough they will glow red. This is a false color IR image. You could use an IR thermometer and "shoot" each one, a wide spread of temperature may indicate one doing a lot worse than the other.

 

Just park the truck in the dark garage and turn the light off and look under.
If your check engine light is on - don't drive it.

If your cats are below efficiency then you will get a code about that eventually
and be unable to smog it.

Otherwise, don't worry about it.
No check engine light - then don't worry.

 

BTW, cats are what cooks out the grease on the front drive shaft u-joints, and causes it to fail. If yours has not been replaced with fully greaseable, time to do that or upgrade the u-joints like the write up in our tech area.

 

You will know if there is a problem when your service engine soon light comes on.

 

'If your cats are below efficiency then you will get a code about that eventually
and be unable to smog it."

Good points, but some of us don't have to meet any smog regulations!

 

True, but if a cat plugs, you need to replace it before it burns your exhaust valves from the back pressure.